Solid high-energy compositions, such as propellants, explosives, gasifiers, or the like, comprise solid particulates, such as fuel particulates and oxidizer particulates, dispersed and immobilized throughout a binder matrix comprising an elastomeric polymer.
Conventional solid composite propellant binders utilize cross-linked elastomers in which prepolymers are cross-linked by chemical curing agents. As outlined in detail in U.S. Pat. No. 4,361,526, there are important disadvantages to using cross-linked elastomers as binders. Cross-linked elastomers must be cast within a short period of time after addition of the curative, which time period is known as the "pot life". Disposal of a cast, cross-linked propellant composition is difficult, except by burning, which poses environmental problems. Furthermore, current state-of-the-art propellant formulations have serious problems that include, but are not limited to, use of nonenergetic binders, high end-of-mix viscosities, thermally labile urethane linkages, and extreme vulnerability to unscheduled detonation.
In view of inherent disadvantages of cross-linked elastomeric polymers as binder matrices, there has been considerable interest in developing thermoplastic elastomers suitable as binder matrices for solid, high-energy compositions. However, many thermoplastic elastomers fail to meet various requirements for propellant formulations, particularly the requirement of being processible below about 120.degree. C., it being desirable that a thermoplastic polymer for use as a binder in a high-energy system have a melting temperature of between about 60.degree. C. and about 120.degree. C. Many thermoplastic elastomers exhibit high melt viscosities which preclude high solids loading and many show considerable creep and/or shrinkage after processing.
The present invention is directed to telechelic polymers that exhibit thermoplastic elastomeric characteristics and which are useful as binders in high-energy compositions. These polymers belong to the general family of materials termed ionomers, i.e., non-polar, often hydrocarbon, polymers which contain bound ionic moieties.
U.S. Pat. No. 3,870,841, the teachings of which are incorporated herein by reference, is directed to polystyrene which is randomly sulfonated. U.S. Pat. Nos. 3,642,728 and 4,184,988, the teachings of which are also incorporated herein by reference, are directed to the sulfonation of olefinically unsaturated elastomers such as butyl rubber and ethylene-propylene-diene rubber (EPDM) at the diene-derived unsaturations which are present at random points along the polymer chain. These works also teach that the resulting sulfonated elastomers can be reacted with alkali metals, alkali metal hydroxides, amines, amine derivatives, etc., to form ionic sulfonic acid salts. The ionic sulfonate moieties tend to aggregate at lower temperatures, which aggregation is destroyed at higher temperatures, whereby these polymers exhibit thermoplastic characteristics. The processes described in these patents yield polymer molecules consisting of two non-functional hydrocarbon chain ends, and one or more elastically effective inner segments which are bounded by the ionic moieties. The topology of the network derived from sulfonate aggregation, regardless of the average number of ion pairs which participate in an aggregate, is such that no covalent branch points exist, and each primary polymer chain contributes two dangling, nonload-bearing ends. If a polymer chain is sulfonated at only one point, it contains no inner segments and cannot participate in the load-bearing funtion. Because of these limitations, the molecular weight of the primary polymer chains, and the level of sulfonation, must be sufficiently high to develop adequate toughness and strength, and this results in undesirably high melt viscosities.
Star-branched, low molecular weight, telechelic ionomers have been shown to make excellent thermoplastic elastomers with very low melt viscosities; J. P. Kennedy et al., ACS Org. Coat. Appl. Polym. Sci. Pro., 46 182 (1982), Y. Mohajer et al., Polym. Bull., 8, 47 (1983), and S. Bagrodia et al., J. Appl. Polym. Sci., 29 (10), 3065 (1984). These works deal with polyisobutylene (PIB)-based ionomers. The structure of a three-arm star PIB ionomer is shown in FIG. 1. The covalent branch point of the star dramatically increases ionomer network connectivity. Because the ionic groups are placed only at the chain ends, the network is free of non-loadbearing chain ends. A disadvantage of these polymers is that PIB is not very resilient at room temperature, and it has a high melt viscosity compared to other elastomers. Another disadvantage is that there is inherently only one ionic group per chain end; thus, there is no way to change ionic concentration independently of molecular weight.
There exists a need for novel thermoplastic elastomers which can be used as binders in high-energy compositions.